Environmental interests and an increasing demand for diesel fuel, especially in Europe, encourage fuel producers to employ more intensively renewable sources available. In the manufacture of diesel fuel based on biological raw materials the main interest has concentrated on vegetable oils and animal fats comprising triglycerides of fatty acids. Long, straight and mostly saturated hydrocarbon chains of fatty acids correspond chemically to the hydrocarbons present in diesel fuels. However, neat vegetable oils display inferior properties, particularly extreme viscosity and poor stability and therefore their use in transportation fuels is limited.
Conventional approaches for converting vegetable oils or other fatty acid derivatives into liquid fuels comprise processes such as transesterification, catalytic hydrotreatment, hydrocracking, catalytic cracking without hydrogen and thermal cracking. Typically triglycerides, forming the main component in vegetable oils, are converted into the corresponding esters by the transesterification reaction with an alcohol in the presence of catalysts. The obtained product is a fatty acid alkyl ester, most commonly fatty acid methyl ester (FAME). Poor low-temperature properties of FAME however limit its wider use in regions with colder climatic conditions. Poor cold flow properties are a result of the straight chain nature of the FAME molecule and thus double bonds are needed in order to create even bearable cold flow properties. Carbon-carbon double bonds and ester groups however decrease the stability of fatty acid esters, which is a major disadvantage of transesterification technology. Further, Schmidt, K., Gerpen J. V.: SAE paper 961086 teaches that the presence of oxygen in esters results in undesired and higher emissions of NOx in comparison to conventional diesel fuels.
Undesired oxygen may be removed from fatty acids or esters by deoxygenation reactions. The deoxygenation of bio oils and fats, which mean oils and fats based on biological material, to hydrocarbons suitable as diesel fuel products, may be carried out in the presence of a catalyst under controlled hydroprocessing conditions, known as hydrotreating or hydrocracking processes.
During hydrodeoxygenation oxogroups are reacted with hydrogen and removed through formation of water. The hydrodeoxygenation reaction requires relatively high amounts of hydrogen. Due to the highly exothermic reactions the control of reaction heat is extremely important. Unnecessary high reaction temperature, insufficient control of reaction temperature or unnecessary low hydrogen availability in the feed stream cause increased formation of unwanted side reaction products and coking of catalyst. Unwanted side reactions, such as cracking, polymerisation, ketonisation, cyclisation and aromatisation decrease the yield and the properties of diesel fraction. Unsaturated feeds and free fatty acids in triglyceridic bio oils may also promote the formation of heavy molecular weight compounds.
U.S. Pat. No. 4,992,605 and U.S. Pat. No. 5,705,722 describe processes for the production of diesel fuel additives by conversion of bio oils into saturated hydrocarbons under hydroprocessing conditions with NiMo and CoMo catalysts. The hydrotreatment operates at high temperatures of 350-450° C. and produces n-paraffins and other hydrocarbons. The product has high cetane number but poor cold properties, which limit the amount of product that can be blended in conventional diesel fuel in summer time and prevent its use during winter time. The formation of heavy compounds with a boiling point above 343° C. was observed, especially when a fatty acid fraction was used as a feed. A lower limit of 350° C. for reaction temperature was concluded as a requirement for trouble-free operation.
A two-step process is disclosed in FI 100248 for producing middle distillates from vegetable oils by hydrogenating fatty acids or triglycerides of vegetable oil origin using commercial sulphur removal catalysts, such as NiMo and CoMo, to give n-paraffins, followed by isomerizing said n-paraffins using metal containing molecule sieves or zeolites to obtain branched-chain paraffins. The hydrotreating was carried out at rather high reaction temperatures of 330-450° C., preferably 390° C. Hydrogenating fatty acids at those high temperatures leads to shortened catalyst life resulting from coking and formation of side products.
EP 1 396 531 describes a process containing at least two steps, the first one being a hydrodeoxygenation step and the second one being a hydroisomerisation step utilizing counter-current flow principle, and biological raw material containing fatty acids and/or fatty acid esters serving as the feedstock. The process comprises an optional stripping step.
Cracking is significant side reaction in isomerisation of n-paraffins. Cracking increases with higher isomerisation conversion (more severe reaction conditions) and decrease the yield of diesel. The severity of isomerisation conditions (isomerisation conversion) controls also the amount of methyl branches formed and their distance from each other and therefore cold properties of bio diesel fraction produced.
FR 2,607,803 describes a process for hydrocracking of vegetable oils or their fatty acid derivatives under elevated pressure to give hydrocarbons and to some extent acid. The catalyst contains metal dispersed on a support. A high reaction temperature of 370° C. did not result in complete conversion and high selectivity of n-paraffins. The product formed contained also some intermediate fatty acid compounds.
Water formation during hydrotreatment mainly results from deoxygenation of triglyceride oxygen by the means of hydrogen (hydrodeoxygenation). Deoxygenation using hydrodeoxygenation conditions is to some extent accompanied by decarboxylation reaction path, described below as reaction A, and decarbonylation reaction path (reaction B1 and B2). Deoxygenation of fatty acid derivatives by decarboxylation and/or decarbonylation reactions forms carbon oxides (CO2 and CO) and aliphatic hydrocarbon chains with one carbon atom less than in the original fatty acid molecule. Thereafter water-gas-shift reaction may balance the concentrations of CO and CO2 (reaction E). Methanation reaction uses hydrogen and forms H2O and methane if it is active during hydrotreatment conditions (reaction D). Hydrogenation of fatty acids gives aliphatic hydrocarbons and water (reaction C). Reaction schemes A-E are described below.
                              Decarboxylation          ⁢                      :                    ⁢                    ⁢                      C            17                    ⁢                      H            35                    ⁢          COOH                →                                            C              17                        ⁢                          H              36                                +                      CO            2                                                      ⁢                  (          A          )                                                  Decarbonylation          ⁢                      :                    ⁢                    ⁢                      C            17                    ⁢                      H            35                    ⁢                      COOH            ÷                          H              2                                      →                                            C              17                        ⁢                          H              36                                +          CO          +                                    H              2                        ⁢            O                                                      ⁢                  (                      B            ⁢                                                  ⁢            1                    )                                                ⁢                                            C              17                        ⁢                          H              35                        ⁢            COOH                    →                                                    C                17                            ⁢                              H                34                                      +            CO            +                                          H                2                            ⁢              O                                                                  ⁢                  (                      B            ⁢                                                  ⁢            2                    )                                                              Hydrogenation            ⁢                          :                        ⁢                        ⁢                          C              17                        ⁢                          H              35                        ⁢            COOH                    +                      3            ⁢                          H              2                                      →                                            C              18                        ⁢                          H              38                                +                      2            ⁢                                                  ⁢                          H              2                        ⁢            O                                                      ⁢                  (          C          )                                                              Methanation            ⁢                          :                        ⁢                        ⁢            CO                    +                      3            ⁢                          H              2                                      →                              CH            4                    +                                    H              2                        ⁢            O                                                      ⁢                  (          D          )                                                              Water            ⁢                          -                        ⁢            Gas            ⁢                          -                        ⁢            shift            ⁢                          :                        ⁢                        ⁢            CO                    +                                    H              2                        ⁢            O                          →                              H            2                    +                      CO            2                                                      ⁢                  (          E          )                    
The feasibility of decarboxylation varies greatly with the type of carboxylic acid or derivative thereof used as the starting material. Alpha-hydroxy, alpha-carbonyl and dicarboxylic acids are activated forms and they are more easily deoxygenated by decarb-reactions, which mean here decarboxylation and/or decarbonylation. Linear aliphatic acids are not activated this way and generally they are difficult to deoxygenate through the decarb-reaction path and they need much more severe reaction conditions.
Decarboxylation of carboxylic acids to hydrocarbons by contacting carboxylic acids with heterogeneous catalysts was suggested by Maier, W. F. et al: Chemische Berichte (1982), 115(2), 808-12. Maier et al tested Ni/Al2O3 and Pd/SiO2 catalysts for decarboxylation of several carboxylic acids. During the reaction the vapors of the reactant were passed through a catalytic bed together with hydrogen. Hexane represented the main product of the decarboxylation of the tested compound heptanoic acid.
U.S. Pat. No. 4,554,397 discloses a process for the manufacture of linear olefins from saturated fatty acids or esters, suggesting a catalytic system consisting of nickel and at least one metal selected from the group consisting of lead, tin and germanium. With other catalysts, such as Pd/C, low catalytic activity and cracking to saturated hydrocarbons, or formation of ketones when Raney-Ni was used, were observed.
Decarboxylation, accompanied with hydrogenation of oxo-compound, is described in Laurent, E., Delmon, B.: Applied Catalysis, A: General (1994), 109(1), 77-96 and 97-115, wherein hydrodeoxygenation of biomass derived pyrolysis oils over sulphided CoMo/γ-Al2O3 and NiMo/γ-Al2O3 catalysts was studied. Diethyldecanedioate was used among others as a model compound and it was observed that the rates of formation of the decarboxylation product, nonane and the hydrogenation product, decane were comparable under hydrotreating conditions (260-300° C., 7 MPa, in hydrogen). The presence of hydrogen sulphide (H2S) in feed promoted the decarboxylation selectivity compared with zero sulphur in feed. Different sulphur levels studied however had no effect on the decarboxylation selectivity of diethyldecanedioate.
Biological raw materials often contain several impurities, such as metal compounds, organic nitrogen, sulphur and phosphorus compounds, which are known catalyst inhibitors and poisons inevitably reducing the service life of catalysts and necessitating more frequent catalyst regeneration or change. Metals in bio oils/fats inevitable build up on catalyst surface and change the activity of catalyst. Metals can promote some side reactions and blocking of active sites of catalysts typically decreases the activity.
Fatty acid composition, size and saturation degree of the fatty acid may vary considerably in feedstock of different origin. Melting point of bio oil or fat is mainly consequence of saturation degree. Fats are more saturated than liquid oils and in this respect need less hydrogen for hydrogenation of double bonds. Double bonds in fatty acid chains contribute also to different kinds of side reactions, such as oligomerisation/polymerization, cyclisation/aromatisation and cracking reactions, which deactivate catalyst, increase hydrogen consumption and reduce diesel yield.
Hydrolysis of triglycerides produces also diglycerides and monoglycerides, which are partially hydrolyzed products. Diglycerides and monoglycerides are surface-active compounds, which can form emulsions and make liquid/liquid separations of water and oil more difficult. Bio oils and fats can also contain other glyceride-like surface-active impurities like phospholipids, suck as lecithin, which have phosphorus in their structures. Phospholipids are gum like materials, which can be harmful for catalysts. Natural oils and fats also contain non-glyceride components. These are among others waxes, sterols, tocopherols and carotenoids, some metals and organic sulphur compounds as well as organic nitrogen compounds. These compounds can be harmful for catalysts or pose other problems in processing.
Plant oils/fats and animal oils/fat may contain free fatty acids, which are formed during processing of oils and fats through hydrolysis of triglycerides. Free fatty acids are a class of problematic components in bio oils and fats, their typical content being between 0 and 30% by weight. Free fatty acids are corrosive in their nature, they can attack the materials of the process unit or catalyst and they can promote side reactions like formation of metal carboxylates in the presence of metal impurities. Due to the free fatty acids contained in bio oils and fats, the formation of heavy molecular weight compounds is significantly increased when compared to triglyceridic bio-feedstock having only low amounts of free fatty acids, typically below 1% by weight.
Deoxygenation of plant oils/fats and animal oils/fats with hydrogen requires rather much hydrogen and at the same time releases significant amount of heat. Heat is produced from the deoxygenation reactions and from double bond hydrogenation. Different feedstocks produce significantly different amounts of reaction heat. The variation in reaction heat produced is mainly dependent of double bond hydrogenation. The average amount of double bonds per triglyceride molecule can vary from about 1.5 to over 5 depending on the source of bio oil or fat.